The GRA is basically zombie legislation at this point but yes, I think we will have challenges based on people having opposite sex ID.
I imagine it won't be a big issue for the very small number of trans people who pass well. It will be an issue for the much larger number who think they pass because they haven't been challenged.
I think (IANAL) the relevant bit is that we can exclude same sex people (TW or TM) from a single sex space if they so closely resemble the opposite sex that there could be a reasonable objection to them being there.
So the gotcha they've been using, what if a bloke trying it on shows up with female ID and says "how do you know I'm not a transman", well even if he were he could still be excluded.
I'm sure someone will be dumb enough to provide the occasion for a court to rule on this.